Three-quarters of a billion dollars might not be enough to pay former NFL players for damage from the bone-jarring, brain-rattling hits they took on the gridiron, a federal judge said Tuesday.

Although the league and some 4,800 former players agreed on a $765 million settlement - a pool which would pay individual players as much as $5 million for neurological problems suffered during their careers - the deal needs the approval of U.S. District Judge Anita Brody. In Tuesday's procedure, Brody expressed concern the money could fall short and the rules governing who is eligible could be too restrictive. She asked for more financial details from the parties, a week after players' lawyers filed a lengthy payout plan.

Brody noted that the plan could have to be stretched to cover nearly 20,000 men for 65 years as more come forward with injuries. The proposed awards would reach $5 million for athletes with amyotrophic lateral sclerosis, or Lou Gehrig's disease; $4 million for a death involving brain trauma; and $3 million for dementia cases.

But if 20,000 players were to divide the entire pool evenly, it would amount to just $38,000 per player.

“I am primarily concerned that not all retired NFL football Players who ultimately receive a qualifying diagnosis, or their related claimants, will be paid,” Brody wrote in papers filed in federal court in Philadelphia.

“Even if only 10 percent of retired NFL football players eventually receive a qualifying diagnosis, it is difficult to see how the monetary award fund would have the funds available over its lifespan to pay all claimants at these significant award levels,” the decision said.

Attorneys for both the league and the former players who brought the suit said they would provide Brody with additional material to show the deal is just.

"We are confident that the settlement will be approved after the Court conducts its due diligence on the fairness and adequacy of the proposed agreement," said lead attorneys Christopher Seeger and Sol Weiss in a statement. "Analysis from economists, actuaries and medical experts will confirm that the programs established by the settlement will be sufficiently funded to meet their obligations for all eligible retired players."

NFL spokesman Brian McCarthy said the league would come back with more information.

“We are confident that the settlement is fair and adequate, and look forward to demonstrating that to the court,” he said.

The lawsuit, filed by the retired players in 2012, contended that the league concealed the dangers of brain injury players faced even as it reaped huge profits. When the settlement was first disclosed, several experts opined that it was a bargain for the league, which is believed to generate annual revenues of $10 billion.

Head injuries among current and former pro football players have made news in recent years, with their dangers underscored by the suicides of players including Jovan Belcher, Junior Seau, Ray Easterling and Dave Duerson. Those deaths cannot be conclusively attributed to blows incurred in football, but medical experts all agree that violent or erratic behavior is consistent with symptoms of a condition tied to repeated hits to the head many players endure in games and practices.

A growing body of academic research shows those hits can lead to chronic traumatic encephalopathy, which can lead to aggression and dementia.

In recent years, the NFL has sought to make the game safer with measures that include banning deliberate helmet-to-helmet hits and requiring teams to keep players who have taken hits to the head off the field and sideline those who have suffered concussions until they recover.